1. Field of the Invention
The present invention relates to an image pickup apparatus for picking up an object image illuminated with light emitted by a light source using an image pickup element such as a CCD.
2. Related Background Art
As a conventional image pickup apparatus using a solid-state image pickup element such as a CCD, a digital copying machine is known.
FIG. 1 shows the arrangement of the conventional digital copying machine. Referring to FIG. 1, the digital copy machine includes an original table glass 82 on which an original 81 is placed, a light source 83, reflection mirrors 84, 85, and 86, a CCD linear image sensor 88, a lens 87 for forming an image of light reflected by the original 81 on the light-receiving surface of the CCD linear image sensor 88, an amplifier 89 for amplifying the output signal from the CCD linear image sensor 88, an A/D converter 90 for converting the amplified CCD output signal into a digital signal, an image processing circuit 91, and a printer 92.
The digital copying machine with the above-mentioned arrangement reads some light components of light emitted from the illumination light source and diffused on the original surface using the CCD linear image sensor, as shown in FIG. 2.
In general, when the light amount exceeds a predetermined value, the output from the CCD linear image sensor 88 is saturated and the sensor 88 cannot obtain an output corresponding to an image. In view of this, the light amount of the light source 82 is set, so that the output from the CCD linear image sensor 88 is not saturated independently of the density of an original placed on the original table glass 82. Therefore, when a normal original is read, the CCD output signal is never saturated.
However, when a glossy body 101 (such as a wrinkled aluminum foil, spectacles with a metal frame, a wristwatch, a gem, or the like) is placed as an original on the original table glass 82, as shown in, e.g., FIG. 3, light emitted by the light source 83 is reflected by the glossy body 101, and forms an image on the CCD linear image sensor 88, as shown in FIG. 4. Note that an original cover 102 is used for pressing an original.
Although the intensity of the reflected light varies depending on the reflectance or position (reflection angle) of the glossy body 101, if the reflected light amount is equal to or larger than a predetermined amount, not only the output from the CCD linear image sensor 88 is saturated, but also blooming may occur in the CCD linear image sensor 88.
The blooming is a phenomenon that a charge generated upon incidence of light exceeding a predetermined amount on a given pixel in a CCD overflows to surrounding pixels. In this case, not only the CCD output is saturated but also signals from the surrounding pixels are destroyed, thus fatally damaging image quality.
A similar problem to that described above is posed in a case shown in FIG. 5 in addition to the above-mentioned case wherein the glossy body is placed on the original table glass. FIG. 5 shows an apparatus which illuminates a film 122 with light emitted by a light source 121, forms an optical image on an original table glass 125 via a lens 123 and a mirror 124, and copies the formed image. Note that a Fresnel lens 87 is used for preventing a light beam incident on the original table glass 125 from diffusing until it forms an image on the CCD linear image sensor 88.
As the film 122, both negative and positive films are available. Since a negative film has a dark base film portion (brightest portion), the light amount for the negative film is set to be considerably larger than that for a positive film to obtain an appropriate output from the CCD linear image sensor 88. Such light amount adjustment normally operates as an AE function, and cannot be controlled by a user.
However, if a negative film has, e.g., a pinhole in this state, light emitted by the light source 121 becomes incident on the CCD linear image sensor 88 via the pinhole. As in the above-mentioned glossy body, blooming may occur in this case.
As a method of solving this problem, the light amount of the light source is decreased to prevent blooming even when a glossy body is placed on the original table glass, or even when a negative film has a pinhole, as described above, and the CCD output is electrically amplified accordingly.
However, with this method, noise is added upon amplification by a circuit, and the S/N ratio is undesirably impaired even when no blooming has occurred.